1. Field of the Invention
The present invention relates to a broadband wireless communication system. More particularly, the present invention relates to an apparatus and method for operating a hierarchical cell in a broadband wireless communication system.
2. Description of the Related Art
A cellular wireless communication system includes a plurality of cells serviced respectively by a plurality of Base Stations (BSs), and each BS communicates wirelessly with Mobile Stations (MSs) within the range of its cell. Thus, an MS receives a service through wireless communication with the BS servicing the cell where the MS is located. However, if the channel condition between the MS and the BS becomes poor due to a geographical condition in the cell, a distance between the MS and the BS, or a movement of the MS, the communication between the MS and the BS may not be able to be performed or the communication performance may be degraded. In particular, if the MS is located in a closed building such as an office or a house, the channel condition between the MS and the BS may become very poor, which causes a propagation shadow area. Thus, the MS located in the propagation shadow area may fail to receive a desired level of service.
An indoor femto BS has been proposed to address the problem of the indoor propagation shadow area. That is, if the conventional BS cannot provide a seamless communication service, a femto BS may be installed to service an indoor propagation shadow area and allow indoor MSs to receive a service through the femto BS, thereby improving an indoor call quality. The femto BS is a small-sized BS that is connected to a mobile communication core network through an indoor broadband network (e.g., x Digital Subscriber Line (xDSL) or cable network) to improve an indoor call quality. Thus, if there is an indoor accessible femto BS, an MS communicates with the femto BS.
As described above, a femto BS may be used to improve the call quality of a user in an indoor shadow area. In addition, a mobile service provider may control the call rates of subscribers variably in such a way as to discount the call rate of a subscriber accessing the femto BS. Unlike an outdoor macro BS having a large coverage with a radius of several kilometers, the femto BS has a small coverage with a radius of several meters and thus has a high probability of being accessed by only a few users. Due to the characteristics of the femto BS, the power for operation of the femto BS must be supplied directly indoors. In this case, if the power consumption of the femto BS is high, it becomes a serious obstacle to the use of the femto BS. What is therefore required is a technique to minimize the power consumption of the femto BS if no user is accessing the femto BS. In addition, if a plurality of femto BSs are installed densely in a common area, each of the femto BSs may interfere with adjacent MSs accessing the macro BS, or femto-BS MSs accessing the other femto BS. What is therefore required is a technique to minimize the interference caused by the use of the femto BSs.